The three families
In a key every chord belongs to a functional family:
- Tonic (T): stability, home. In C: C (I), Am (vi), Em (iii).
- Subdominant / predominant (S): moving away, preparation. F (IV), Dm (ii).
- Dominant (D): tension that asks for the tonic. G / G7 (V), B° (vii°).
Chords of the same family share notes: Am has two notes in common with C (C and E), Dm two with F (F and A), B° three with G7 (B D F).
The T → S → D → T cycle
It is the basic harmonic phrase: C – F – G7 – C, or C – Am – Dm – G7 – C. The engine is the tritone B–F of G7: B rises to C, F falls to E. The reverse movement (D → S, that is, G → F) is typical of rock and blues and weakens the cadence.
In minor
In A minor: tonic Am (i), F (VI); subdominant Dm (iv), Bø (iiø); dominant E/E7 (V, from the harmonic minor) and G#°7 (vii°7). The natural v (Em) and the VII (G) have a weak dominant function: it is the “modal” sound of minor pop.
Functions and modern pop
In a four-chord loop the function depends on context: in Am F C G the Am sounds like the tonic, but just start the loop from C (C G Am F) and the center shifts. Knowing the functions lets you substitute chords (C → Am, F → Dm) without breaking the path.
Real example
“La Bamba” (Ritchie Valens, 1958) and “Twist and Shout” (in the Beatles' version, 1963) repeat I–IV–V: in C that would be C F G. Three chords, one per function, and it's already a song.